arXiv subjects

Felix von Oppen

Publications and source records attributed to Felix von Oppen.

At least 127 records · Page 7Linked to original sources

arXiv2013

Electron-phonon interaction on the surface of a 3D topological insulator

We analyze the role of a Rayleigh surface phonon mode in the electron-phonon interaction at the surface of a 3D topological insulator. A strong renormalization of the phonon dispersion, leading eventually to the disappearance of Rayleigh phonons, is ruled out in the ideal case of a continuum long-wavelength limit, which is only justified if the surface is clean and defect free. The absence of backscattering for Dirac electrons at the Fermi surface is partly responsible for the reduced influence of the electron-phonon interaction. A pole in the dielectric response function due to the Rayleigh phonon dispersion could drive the electron-electron interaction attractive at low frequencies. However, the average pairing interaction within the weak coupling approach is found to be too small to induce a surface superconducting instability.

cond-mat.mes-hall↗

arXiv2013

Adiabatic quantum motors

When parameters are varied periodically, charge can be pumped through a mesoscopic conductor without applied bias. Here, we consider the inverse effect in which a transport current drives a periodic variation of an adiabatic degree of freedom. This provides a general operating principle for adiabatic quantum motors, for which we develop a comprehensive theory. We relate the work performed per cycle on the motor degree of freedom to characteristics of the underlying quantum pump and discuss the motor's efficiency. Quantum motors based on chaotic quantum dots operate solely due to quantum interference, motors based on Thouless pumps have ideal efficiency.

cond-mat.mes-hall↗

arXiv2013

Real-Space Tailoring of the Electron-Phonon Coupling in Ultra-Clean Nanotube Mechanical Resonators

The coupling between electrons and phonons is at the heart of many fundamental phenomena in physics. In nature, this coupling is generally predetermined for both, molecules and solids. Tremendous advances have been made in controlling electrons and phonons in engineered nanosystems, yet, control over the coupling between these degrees of freedom is still widely lacking. Here, we use a new generation of carbon nanotube devices with movable ultra-clean single and double quantum dots embedded in a mechanical resonator to demonstrate the tailoring of the interactions between electronic and mechanical degrees of freedom on the nanoscale. Exploiting this tunable coupling, we directly image the spatial structure of phonon modes and probe their parity in real space. Most interestingly, we demonstrate selective coupling between individual mechanical modes and internal electronic degrees of freedom. Our results open new vistas for engineering bulk quantum phenomena in a controlled nanoscale setting and offer important new tools for entangling the electronic and mechanical degrees of freedom at the quantum level.

cond-mat.mes-hall↗

arXiv2013

Out-of-equilibrium catalysis of chemical reactions by electronic tunnel currents

We present an escape rate theory for current-induced chemical reactions. We use Keldysh nonequilibrium Green's functions to derive a Langevin equation for the reaction coordinate. Due to the out of equilibrium electronic degrees of freedom, the friction, noise, and effective temperature in the Langevin equation depend locally on the reaction coordinate. As an example, we consider the dissociation of diatomic molecules induced by the electronic current from a scanning tunnelling microscope tip. In the resonant tunnelling regime, the molecular dissociation involves two processes which are intricately interconnected: a modification of the potential energy barrier and heating of the molecule. The decrease of the molecular barrier (i.e. the current induced catalytic reduction of the barrier) accompanied by the appearance of the effective, reaction-coordinate-dependent temperature is an alternative mechanism for current-induced chemical reactions, which is distinctly different from the usual paradigm of pumping vibrational degrees of freedom.

cond-mat.mes-hall↗

arXiv2013

Signatures of topological phase transitions in mesoscopic superconducting rings

We investigate Josephson currents in mesoscopic rings with a weak link which are in or near a topological superconducting phase. As a paradigmatic example, we consider the Kitaev model of a spinless p-wave superconductor in one dimension, emphasizing how this model emerges from more realistic settings based on semiconductor nanowires. We show that the flux periodicity of the Josephson current provides signatures of the topological phase transition and the emergence of Majorana fermions situated on both sides of the weak link even when fermion parity is not a good quantum number. In large rings, the Majorana fermions hybridize only across the weak link. In this case, the Josephson current is h/e periodic in the flux threading the loop when fermion parity is a good quantum number but reverts to the more conventional h/2e periodicity in the presence of fermion-parity changing relaxation processes. In mesoscopic rings, the Majorana fermions also hybridize through their overlap in the interior of the superconducting ring. We find that in the topological superconducting phase, this gives rise to an h/e-periodic contribution even when fermion parity is not conserved and that this contribution exhibits a peak near the topological phase transition. This signature of the topological phase transition is robust to the effects of disorder. As a byproduct, we find that close to the topological phase transition, disorder drives the system deeper into the topological phase. This is in stark contrast to the known behavior far from the phase transition, where disorder tends to suppress the topological phase.

cond-mat.mes-hall↗

arXiv2013

Photocurrent Response of Topological Insulator Surface States

We study the photocurrent response of topological insulator surface states to circularly polarized light for arbitrary oblique incidence. We describe the surface states within a Dirac model, including several perturbations such as hexagonal warping, nonlinear corrections to the mode velocity, and applied magnetic fields. We find that the photogalvanic current is strongly suppressed for the usual orbital coupling, prompting us to include the weaker Zeeman coupling. We find that the helicity-independent photocurrent dominates over the helicity-dependent contributions.

cond-mat.mes-hall↗

arXiv2012

Enhanced zero-bias Majorana peak in disordered multi-subband quantum wires

A recent experiment [Mourik et al., Science 336, 1003 (2012)] on InSb quantum wires provides possible evidence for the realization of a topological superconducting phase and the formation of Majorana bound states. Motivated by this experiment, we consider the signature of Majorana bound states in the differential tunneling conductance of multi-subband wires. We show that the weight of the Majorana-induced zero-bias peak is strongly enhanced by mixing of subbands, when disorder is added to the end of the quantum wire. We also consider how the topological phase transition is reflected in the gap structure of the current-voltage characteristic.

cond-mat.mes-hall↗

arXiv2012

Scattering theory of adiabatic reaction forces due to out-of-equilibrium quantum environments

The Landauer-Buettiker theory of mesoscopic conductors was recently extended to nanoelectromechanical systems. In this extension, the adiabatic reaction forces exerted by the electronic degrees of freedom on the mechanical modes were expressed in terms of the electronic S-matrix and its first non-adiabatic correction, the A-matrix. Here, we provide a more natural and efficient derivation of these results within the setting and solely with the methods of scattering theory. Our derivation is based on a generic model of a slow classical degree of freedom coupled to a quantum-mechanical scattering system, extending previous work on adiabatic reaction forces for closed quantum systems.

cond-mat.mes-hall↗

arXiv2012

Magneto-Josephson effects in junctions with Majorana bound states

We investigate 1D quantum systems that support Majorana bound states at interfaces between topologically distinct regions. In particular, we show that there exists a duality between particle-hole and spin degrees of freedom in certain spin-orbit-coupled 1D platforms such as topological insulator edges. This duality results in a spin analogue of previously explored `fractional Josephson effects'---that is, the spin current flowing across a magnetic junction exhibits 4π-periodicity in the relative magnetic field angle across the junction. Furthermore, the interplay between the particle-hole and spin degrees of freedom results in unconventional magneto-Josephson effects, such that the Josephson current is a function of the magnetic field orientation with periodicity 4π.

cond-mat.mes-hall↗

arXiv2012

Relaxation and edge reconstruction in integer quantum Hall systems

The interplay between the confinement potential and electron-electron interactions causes reconstructions of Quantum Hall edges. We study the consequences of this edge reconstruction for the relaxation of hot electrons injected into integer quantum Hall edge states. In translationally invariant edges, the relaxation of hot electrons is governed by three-body collisions which are sensitive to the electron dispersion and thus to reconstruction effects. We show that the relaxation rates are significantly altered in different reconstruction scenarios.

cond-mat.mes-hall↗

arXiv2012

Inelastic electron backscattering in a generic helical edge channel

We evaluate the low-temperature conductance of a weakly interacting one-dimensional helical liquid without axial spin symmetry. The lack of that symmetry allows for inelastic backscattering of a single electron, accompanied by forward-scattering of another. This joint effect of weak interactions and potential scattering off impurities results in a temperature-dependent deviation from the quantized conductance, $δG \propto T^4$. In addition, $δG$ is sensitive to the position of the Fermi level. We determine numerically the parameters entering our generic model for the Bernevig-Hughes-Zhang Hamiltonian of a HgTe/CdTe quantum well in the presence of Rashba spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2012

Current-induced forces in mesoscopic systems: a scattering matrix approach

Nanoelectromechanical systems are characterized by an intimate connection between electronic and mechanical degrees of freedom. Due to the nanoscopic scale, current flowing through the system noticeably impacts the vibrational dynamics of the device, complementing the effect of the vibrational modes on the electronic dynamics. We employ the scattering matrix approach to quantum transport to develop a unified theory of nanoelectromechanical systems out of equilibrium. For a slow mechanical mode, the current can be obtained from the Landauer-Büttiker formula in the strictly adiabatic limit. The leading correction to the adiabatic limit reduces to Brouwer's formula for the current of a quantum pump in the absence of the bias voltage. The principal result of the present paper are scattering matrix expressions for the current-induced forces acting on the mechanical degrees of freedom. These forces control the Langevin dynamics of the mechanical modes. Specifically, we derive expressions for the (typically nonconservative) mean force, for the (possibly negative) damping force, an effective "Lorentz" force which exists even for time reversal invariant systems, and the fluctuating Langevin force originating from Nyquist and shot noise of the current flow. We apply our general formalism to several simple models which illustrate the peculiar nature of the current-induced forces. Specifically, we find that in out of equilibrium situations the current induced forces can destabilize the mechanical vibrations and cause limit-cycle dynamics.

cond-mat.mes-hall↗

arXiv2012

Large current noise in nanoelectromechanical systems close to continuous mechanical instabilities

We investigate the current noise of nanoelectromechanical systems close to a continuous mechanical instability. In the vicinity of the latter, the vibrational frequency of the nanomechanical system vanishes, rendering the system very sensitive to charge fluctuations and, hence, resulting in very large (super-Poissonian) current noise. Specifically, we consider a suspended single-electron transistor close to the Euler buckling instability. We show that such a system exhibits an exponential enhancement of the current noise when approaching the Euler instability which we explain in terms of telegraph noise.

cond-mat.mes-hall↗

arXiv2012

Current-induced switching in transport through anisotropic magnetic molecules

Anisotropic single-molecule magnets may be thought of as molecular switches, with possible applications to molecular spintronics. In this paper, we consider current-induced switching in single-molecule junctions containing an anisotropic magnetic molecule. We assume that the carriers interact with the magnetic molecule through the exchange interaction and focus on the regime of high currents in which the molecular spin dynamics is slow compared to the time which the electrons spend on the molecule. In this limit, the molecular spin obeys a non-equilibrium Langevin equation which takes the form of a generalized Landau-Lifshitz-Gilbert equation and which we derive microscopically by means of a non-equilibrium Born-Oppenheimer approximation. We exploit this Langevin equation to identify the relevant switching mechanisms and to derive the current-induced switching rates. As a byproduct, we also derive S-matrix expressions for the various torques entering into the Landau-Lifshitz-Gilbert equation which generalize previous expressions in the literature to non-equilibrium situations.

cond-mat.mes-hall↗

arXiv2012

Topological superconducting phases in disordered quantum wires with strong spin-orbit coupling

Zeeman fields can drive semiconductor quantum wires with strong spin-orbit coupling and in proximity to s-wave superconductors into a topological phase which supports end Majorana fermions and offers an attractive platform for realizing topological quantum information processing. Here, we investigate how potential disorder affects the topological phase by a combination of analytical and numerical approaches. Most prominently, we find that the robustness of the topological phase against disorder depends sensitively and non-monotonously on the Zeeman field applied to the wire.

cond-mat.mes-hall↗

arXiv2012

Manipulating Majorana fermions using supercurrents

Topological insulator edges and spin-orbit-coupled quantum wires in proximity to s-wave super- conductors can be tuned through a topological quantum phase transition by a Zeeman field. Here we show that a supercurrent flowing in the s-wave superconductor also drives such a transition. We propose to use this new mechanism to generate and manipulate Majorana fermions that localize at domain walls between topological and nontopological regions of an edge or wire. In quantum wires, this method carries the added benefit that a supercurrent reduces the critical Zeeman field at which the topological phase appears.

cond-mat.mes-hall↗

arXiv2011

Adiabatic manipulations of Majorana fermions in a three-dimensional network of quantum wires

It has been proposed that localized zero-energy Majorana states can be realized in a two-dimensional network of quasi-one-dimensional semiconductor wires that are proximity-coupled to a bulk superconductor. The wires should have strong spin-orbit coupling with appropriate symmetry, and their electrons should be partially polarized by a strong Zeeman field. Then, if the Fermi level is in an appropriate range, the wire can be in a topological superconducting phase, with Majorana states that occur at wire ends and at $Y$ junctions, where three topological superconductor segments may be joined. Here we generalize these ideas to consider a three-dimensional network. The positions of Majorana states can be manipulated, and their non-Abelian properties made visible, by using external gates to selectively deplete portions of the network, or by physically connecting and redividing wire segments. Majorana states can also be manipulated by reorientations of the Zeeman field on a wire segment, by physically rotating the wire about almost any axis, or by evolution of the phase of the order parameter in the proximity-coupled superconductor. We show how to keep track of sign changes in the zero-energy Hilbert space during adiabatic manipulations by monitoring the evolution of each Majorana state separately, rather than keeping track of the braiding of all possible pairs. This has conceptual advantages in the case of a three-dimensional network, and may be computationally useful even in two dimensions, if large numbers of Majorana sites are involved.

cond-mat.supr-con↗

arXiv2011

Fictitious gauge fields in bilayer graphene

We discuss the effect of elastic deformations on the electronic properties of bilayer graphene membranes. Distortions of the lattice translate into fictitious gauge fields in the electronic Dirac Hamiltonian which are explicitly derived here for arbitrary elastic deformations. We include gauge fields associated to intra- as well as inter-layer hopping terms and discuss their effects in different contexts. As a first application, we use the gauge fields in order to study the recently predicted strain-induced Lifshitz transition for the Fermi surface at low energy. As a second application, we discuss the electron-phonon coupling induced by the fictitious gauge fields and analyse its contribution to the electrical resistivity of suspended bilayer membranes. Of special interest is the appearance of a linear coupling for flexural modes, in stark contrast to the case of monolayer graphene. This new coupling channel is shown to dominate the temperature-dependent resistivity in suspended samples with low tension.

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